1. Field of the Invention
The present invention relates to a semiconductor device provided with a thin-film transistor, and to a method for manufacturing the same.
2. Description of the Related Art
Semiconductor devices in which a thin-film transistor (Thin Film Transistor: also referred to hereinafter as TFT) is formed on a glass substrate or other insulating substrate are widely known as devices for driving active-matrix-type liquid crystal flat-panel displays, for example. Recent miniaturization and addition of multifunction capability to such semiconductor devices has necessitated TFTs having high durability and reliability.
A TFT has therefore been developed that is provided with a low-concentration drain region (Lightly Doped Drain: also referred to hereinafter as LDD region) in order to achieve high durability and reliability in a TFT. However, when a TFT provided with this type of LDD structure is manufactured, impurity ions are implanted into a semiconductor layer over a gate insulating film using the gate electrode as the mask after the gate insulating film and gate electrode are formed on the semiconductor layer. The gate insulating film therefore degrades due to the passage of the impurity ions, and it is sometimes impossible to ensure adequate reliability even when an LDD region is provided. These effects are thought to result from the formation of an element (carrier trap) that captures a carrier inside the gate insulating film through which ions have passed.
The gate insulating film in a TFT is usually left on the entire surface of the substrate without being patterned. This is done in order to maintain insulating properties between a semiconductor layer patterned as islands and the layer formed thereon. The ions implanted when the LDD region is formed are therefore implanted into a portion that protrudes from the area directly under the gate electrode in the gate insulating film. The gate insulating film in this portion is therefore damaged, and its characteristics are adversely affected. A hydrogen plasma treatment is performed on the semiconductor layer as needed, but the gate insulating film is also damaged by the hydrogen plasma treatment, and its characteristics are adversely affected. The characteristics of the gate insulating film are also compromised by contamination with metal.
Furthermore, the gate insulating film is usually formed by plasma CVD (Chemical Vapor Deposition) or sputtering when a TFT is formed on a glass substrate, but a fixed charge is easily introduced into the gate insulating film in these methods. This fixed charge in the gate insulating film affects the impurity concentration of the LDD and alters the characteristics of the TFT.
A technique is disclosed in Japanese Patent No. 3177360 whereby the portion that protrudes from the area directly under the gate electrode in the gate insulating film is removed partway in the film thickness direction after the gate electrode is formed. According to the abovementioned publication, the portion of the gate insulating film that is damaged by ion implantation can thereby be removed.
A technique is also disclosed in Japanese Laid-open Patent Application No. 10-27911 whereby a base insulating film, a gate insulating film, or another insulating film is formed on an insulating substrate, after which the insulating film is immersed in an electrolyte solution. According to Japanese Laid-open Patent Application No. 10-27911, fixed charges in the insulating film can thereby be removed, and the electrical characteristics of the TFT can be improved.
However, the conventional techniques described above have such problems as those described below. The TFT characteristics cannot be adequately stabilized merely by removing the portion of the gate insulating film that protrudes from the area directly under the gate electrode partway in the film thickness direction as described in Japanese Patent No. 3177360. The stabilizing effects on the characteristics of the TFT are also inadequate even when the base insulating film, gate insulating film, or other insulating film is immersed in an electrolyte solution as described in Japanese Laid-open Patent Application No. 10-27911.